You've apparently done reading on this subject so I doubt there is much I can contribute on "damaging metabolism" and "set point" theory.
With or without WLS we need a minimum number of calories to support a healthy body. 800-1000, or even 1200 calories as a daily norm seems low, except the most extraordinarily sedentary life styles.
Health.gov puts average healthy calorie intake generally [age/lifestyle breakdown at site] for women (5'4", 124lbs) to be 1600-2400 calories and for men (5'10", 154lbs) 2000-3000 calories.
An RNY or DS patient, who absorbs fewer calories than we eat would need some amount higher than the averages above. Of course, the calories not absorbed would at best be a WAG (Wild Arse Guess).
Now, honestly the Health.gov calorie requirements seem high to me, but who am I to argue with Health.gov.
Good luck
Tek